The present invention relates to medical imaging. More particularly, the invention relates to magnetic resonance imaging (MRI) systems and methods using an arrival time correction (ATC) for dynamic susceptibility contrast (DSC) based permeability imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0) applied along, for example, a Z axis of a Cartesian coordinate system, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A NMR signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image or produce a spectrum.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space”. Typically, a region to be imaged is scanned by a sequence of measurement cycles in which gradients vary according to the particular localization method being used. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) that have the same direction as the polarizing field B0, but which have a gradient along the respective x, y, and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified. The acquisition of the NMR signals samples is referred to as sampling k-space, and a scan is completed when enough NMR cycles are performed to adequately sample k-space. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MRI techniques can be used to image the blood-brain barrier (BBB), for example, using dynamic contrast enhanced (DCE), T1-weighted imaging studies. Such methods involve calculation of a measure of permeability, Ktrans. Although DCE MRI has been shown to be a robust research tool, it has yet to become part of standard clinical practice. In part this is due to the time-consuming process of acquiring the images required for generating permeability measures with DCE MRI.
Conversely, dynamic susceptibility contrast (DSC) MRI is a routinely acquired imaging technique most commonly used in ischemic stroke patients or brain tumor patients. In brain tumor patients DSC MRI is used to measure cerebral blood volume (CBV) of the tumor as this has been associated with tumor grade. However, leakage of contrast due to BBB disruption can lead to an underestimation of CBV.
A method for contrast leakage has been described (Zaharchuk G (2007) Theoretical basis of hemodynamic MR imaging techniques to measure cerebral blood volume, cerebral blood flow, and permeability. AJNR Am J Neuroradiol 28: 1850-1858) and applied to brain tumor patients (Boxerman J L, Schmainda K M, Weisskoff R M (2006) Relative cerebral blood volume maps corrected for contrast agent extravasation significantly correlate with glioma tumor grade, whereas uncorrected maps do not. AJNR Am J Neuroradiol 27: 859-867). In order to correct for BBB disruption, a measure of permeability is extracted from the DSC MRI acquisition. This approach generates a measure that has been labeled K2, which is related to Ktrans.
DSC MRI is routinely collected on acute stroke patients at many large academic medical centers as part of the evaluation for treatment. In this setting it is referred to as perfusion weighted imaging (PWI) and provides information about the blood flow to the brain.
Several groups have attempted to extract permeability information from PWI in stroke. However, the approach used in these attempts, which assumes uniform perfusion of the brain, can be subject to error when applied to patients with perfusion deficits, such as acute stroke patients. The delay in contrast delivery to areas of hypoperfusion makes calculation of K2 inaccurate.
Therefore, it would be desirable to have a system and method for extending PWI to all clinical settings patients with perfusion deficits, including those with acute stroke, which often are in particular need of dynamic imaging studies.